The most influential man of the 20th century, albeit negatively,was Adolph Hitler.
Now before anyone comes unglued,let me explain.
His actions resulted in a war that the nations of the world still have not recovered from.
His actions almost totally destroyed a whole religion.
His actions led to the development of some of the most terrible weapons ever created.
His actions led to the destruction of thousands of years of art,literature,architecture,and other achievements.
His actions led to the creation of both the Warsaw Pact and Nato,which split Europe for almost half a century.
On the positive,his actions led to the creation of the UN,the development of modern combat medicine that is still saving lives on the battlefield,worldwide recognition that there must be rules of war and the war crimes acts that allowed his henchmen to be tried.
So,like it or not,he was the most influential man of the 20th century.